This invention relates to a master cylinder and more particularly, to a master cylinder having a fluid pressure control valve mechanism for adjusting the fluid pressure distribution between front and rear wheel brake systems in a vehicle such as an automobile, the brake systems being of the type in which the pistons are slidable in the control valve mechanism without being obstructed by trapped fluid pressure.
A variety of master cylinders having a fluid pressure control valve are known and one of the prior art master cylinders is the so-called "two system" brake mechanism in which a front wheel brake and one fluid pressure chamber of the master cylinder constitute one fluid pressure system and a rear wheel brake and the other fluid pressure chamber of the master cylinder constitute the other fluid pressure system, the master cylinder having a fluid pressure control valve mechanism integrally formed therewith to control the pressure in front and rear brakes to a predetermined level. In one of the prior art fluid pressure control mechanisms referred to hereinabove, a stopped piston having a larger diameter portion and a smaller diameter portion and a differential piston having a diameter smaller than that of the larger diameter portion of the piston but larger than that of the smaller diameter portion of the piston are slidably received in end-to-end abutting relationship in the body of the control valve mechanism, a communication passage extends between the other pressure chamber of the master cylinder and a rear wheel brake and a valve for opening and closing said communication passage are provided in the stepped piston. A biasing force is applied to one end of the stepped piston by the rear wheel brake fluid pressure and the pressure of a spring and the front wheel brake fluid pressure is applied to one end of the differential piston whereby the fluid pressure distribution between the rear wheel and front wheel brake systems is so controlled that when a predetermined braking force is applied, the fluid pressure on the rear wheel brake is smaller than that of the front wheel brake. However, in this prior art fluid pressure control valve mechanism, when oil under pressure from the passage between the master cylinder and brake systems leaks into and is trapped in the annular space defined between the stepped piston and the differential piston, the slidable movement of the differential piston is impeded to such a degree that the control valve mechanism can not function satisfactorily.